In the fabrication of high performance glass ceramic chip carriers with copper based internal wiring in the fabrication of high circuit density electronic modules, it is generally observed that there is higher incidence of ceramic chipping and surface imperfections occurring in the manufacturing operations and testing procedures which is caused by the inherent fragility and lower strength of glass ceramic-copper than alumina ceramic-molybdenum based substrates. When such defects appear in the seal band perimeter region designed for protective cap attachment in the case of non-hermetic modules or in the flange area in the case of the hermetic modules and when the defect dimensions do not meet the acceptance specification criteria, the product is rejected at the final inspection. For the seal integrity in the top surface protective cap attachment, it is critical that the top surface metal (TSM) flange or seal band area be free of such defects in order to assure that in the module assembly, the silicon devices remain protected from any detrimental effect of environmental exposure during the long-term functional performance of the electronic device. Any defects or imperfections at the seal band area can interfere with the sealing process causing problem with seal integrity and long term functional reliability.
Considering the generally observed product yield loss due to seal region chipped ceramic in manufacturing adding to the overall product cost and the assembled product reliability issues, it is highly desirable to have a uv-curable composition and a ceramic chip repair method that is practical, efficient, low cost, reliable, and extendable to various ceramic chip carriers so as to recover/reclaim high performance glass ceramic products. Various requirements for the material composition and the defect repair method include good adhesion to ceramic; good flow properties suitable for chip fill without introducing voids; low shrinkage upon cure; high thermal stability of the cured polymer deposit filling the chipped ceramic area such that it is compatible with bond and assembly processing which includes solder flux exposure; high temperature chip join solder reflow requiring 350-365° C. peak temperature for 97/3 lead/tin alloy (97% Pb/3% Sn) solder bumps; flux residue cleaning process; good mechanical properties, and the defect repaired region having no detrimental effect on the seal integrity. It is also desirable that the uv-cured polymer materials used for repair have compatibility with lead-free solder joining processes for silicon devices to chip carriers. Other requirements for the material in defect repair processes is the ability to fill defects 2-30 mils deep in a ceramic chip, preferably 5-20 mils deep, without drooping during filling or introducing voids or blistering when it is subjected to uv curing followed by thermal bake.
A number of uv curable compositions for use as protective coatings and PCB solder mask in low temperature soldering processes for component attachment have been described in the literature and many such uv curable materials are commercially available. In the solder mask application, for example, the solder mask composition covers the selected areas of the printed wiring board (PWB) circuitry allowing selective soldering of the areas which are covered by the solder mask and thus prevents solder bridges during component soldering process. The solder mask is typically kept permanently in place to provide long term insulation and protection against mechanical and environmental damage.
U.S. Pat. No. 4,533,445 (Orio et al.) describes a uv-curable composition with good adhesion to copper and Pb—Sn solder, comprising an epoxy resin, an acrylated monomer/oligomer, a photoinitiator, and an inorganic filler. These compositions are screen printed onto circuit boards.
U.S. Pat. No. 4,544,623 (Audykowski et al.) discloses photosensitive coating compositions and use thereof for protective purposes such as a solder mask on a PWB. These compositions contain organic solvents and are based on photosensitive epoxide resin carrying an ethylenically unsaturated group bound as a side chain or molecular chain as chalcone group, a customary curing agent, a cure accelerator and an organic or inorganic filler.
U.S. Pat. No. 4,902,578 (Kerr, III et al.) discloses a solvent-free uv-curable composition comprising an elastomeric polymer, bifunctional monomer, a multifunctional acrylic monomer, and a photoinitiator, the polymeric binder is acrylonitrile-butadiene which includes amino and hydroxyl groups. These compositions are used as transparent protective coatings on thermoplastic substrates such as polycarbonate sheets.
The uv-curable compositions described in the prior art are generally based on epoxide resin, epoxide/acrylic resins or epoxide/acrylated urethanes, some of which also carry organic solvents. The problem with epoxy based crosslinked thermoset materials as protective coatings on organic, inorganic, and metal surfaces is that these are high stress, high modulus and generally brittle materials which also have the problem of high moisture uptake. The presence of organic solvents in uv-curable formulations also has several problems, for example, solvent outgassing during cure, flammability, chemical safety, and health hazard when using low boiling solvents. In addition some of the solvents are classified as VOC (Volatile Organic Compounds) or HAPs (Hazardous Air Pollutants), which are subject to environmental regulations requiring strict control of volatile emissions using special emission control devices, and in some cases record keeping and regular reporting of emissions.
U.S. Pat. No. 6,682,872 discloses solvent-free, uv-curable polymer-filler composite formulations and a method of use in repairing chipped glass ceramic defects in thin film ceramic chip carriers to provide manufacturing product cost reduction by reclaiming defective modules and thereby provide yield loss recovery. The polymer-filler compositions employed for defect repair are prepared by dispersing an inorganic filler, specifically, magnesium silicate (Talc), silane modified silica, and/or organophilic nanoclay in a polymer matrix comprising a blend of reactive monomers and a low Tg low stress reactive oligomer, and a photoinitiator. Prior to repairing the chipped site at the seal band region, the ceramic substrates are precleaned by solvent rinse, for example with 2-propanol (IPA), and oven dried to remove adsorbed solvent. The uv-curable polymer-filler dispersion is then dispensed onto the chipped region to fill the defect followed by uv curing and thermal bake up to at least 270-285° C. after which the cured polymer fill is planarized when found necessary by sanding or shaving such that the polymer composite repaired region is coplanar with the adjacent ceramic topography. This method was successfully used to repair ceramic chip defects in the perimeter region of hermetic ceramic modules having multiple high circuit density thin film wiring structure with polyimide insulator.
It was found that in the application of these compositions and the defect repair process for ceramic chip repair in non-hermetic substrates having un-polished surfaces and higher porosity of ceramic, occasional adhesion loss and incidence of crazing in the repaired site polymer deposit was observed upon multiple high temperature chip join reflows with solder reflow peak temperature being 350-365° C. These differences between the polished vs. the un-polished substrates with higher porosity of ceramic may be due to trace moisture/other volatiles being trapped in the capillaries of the later type of ceramic resulting in outgassing during high temperature thermal reflows causing disruption of bond integrity at the ceramic/polyfill interphase region. Other problems such as crazing of the polyfill material in the repaired site may be process related caused by embedding of polishing media particulate in the polymer creating stress nuclei during sanding to obtain planarization of the repaired site.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide improved uv-curable compositions for use in electronics assembly and an improved method of using these and other uv-curable compositions in electronics assembly.